1. Field of the Invention
The present invention relates to a method of manufacturing a photomask which is used in lithographic process during fabrication of a semiconductor device or the like.
2. Description of the Prior Art
A photomask for use in lithographic process in general includes a predetermined transfer pattern which consists of a portion transparent to illuminated light and a portion opaque to the illuminated light. The transfer pattern formed on the photomask is projected by a lens system onto a target substrate which includes a photosensitive material layer, and eventually transferred onto the photosensitive material layer.
FIG. 9A is a cross sectional view of a conventional photomask. On a transparent substrate 1 made of glass or other suitable material, light shielding regions 2 made of Cr, MoSi and the like are formed. A transfer pattern is formed by the light shielding regions 2.
In a projected image of such a photomask, as seen in FIG. 9B which shows the amplitude distribution of transmitted light, due to diffraction, light passed through the transparent substrate 1 floods into light shielding areas under the light shielding regions 2. Since an actual light intensity is a square of the amplitude, the distribution of the light intensity is as shown in FIG. 9C. Thus, in the conventional photomask, light is present in the light shielding areas which are created by the light shielding regions 2, which deteriorates the pattern transfer resolution and hence the transfer accuracy of fine patterns.
As a method of preventing such diffraction-induced deterioration in the resolution, the phase shift method is well known in the art. The phase shift method requires that transparent parts T1, T2 . . . and light shielding parts S1, S2, S3 . . . are alternately arranged in a photomask and that a phase shifting region 3 is disposed on every other transparent part as shown in FIG. 10A. That is, in the transparent part T2, the phase shifting region 3 is formed on the transparent substrate 1 between adjacent light shielding regions 2. The phase shifting regions 3 each have a thickness which gives rise to 180-degree phase difference between light therethrough and light passed through the transparent part not having a phase shifting region.
Hence, light passed through adjacent transparent parts and flooding under the light shielding part therebetween interfere and cancel each other. For instance, light passed through the transparent parts T1 and T2 and flooding under the light shielding part S2 cancel each other by interference therebetween under the light shielding part S2. This enhances the resolution of the photomask.
FIG. 11A is a plan view showing the photomask of FIG. 10A as modified in that three light shielding regions 2 of the same dimensions are arranged parallel to and equidistant from each other. FIG. 11B shows the distribution of the amplitude of light taken along the line I--I of FIG. 11A. FIG. 11C shows the distribution of the intensity of the light. FIG. 11D shows the distribution of the amplitude of light taken along the line II--II of FIG. 11A and FIG. 11E shows the distribution of the intensity of the light.
As obvious from FIGS. 11A to 11E, in this photomask, each phase shifting region 3 directly contacts the transparent substrate 1 at its periphery, i.e., a region 3a. A light beam through the phase shifting regions 3 is 180 degrees out of phase from a light beam through the transparent substrate 1 (FIGS. 11B and 11D), and therefore, the two transmitted light beams interfere and, cancel each other in the border region 3a. Hence, the light intensity at the border region 3a drops to zero as shown in FIGS. 11C and 11B.
Because of this, when the mask pattern of this photomask is transferred and developed on a photo resist layer which is disposed on a substrate-to-be-processed, if the photo resist layer 5 on the substrate-to-be-processed 4 is a positive type resist, residual resist as shown by the dash-and-dot line 6 in FIG. 12 will be created at a region in which the resist should be removed. Conversely, if the photo resist layer 5 is a negative type resist, the resist will be removed at a region in which it should remain. Hence, use of the phase shifting regions 3 as above is prohibited in some cases since a mask pattern for actual use in fabrication of a semiconductor device in general includes such isolated island-like shaped light shielding regions 2 as those shown in FIG. 11A.